1. Field of the Invention
This invention relates to an electrical connector including means for preventing or suppressing an arc when power contacts are disconnected or separated while they carry substantial power or electrical current. This invention also relates to an electrical connector that preferentially uses a positive temperature coefficient resistor shunted between contacts that are disconnected sequentially so that voltage and current will be below a threshold at which arcing might occur, when each contact is separated from a mating contact.
2. Description of the Prior Art
Contacts carrying significant amounts of power will arc when disconnected. The amount of arc damage experienced by the contacts depends on their physical structure, the load current, the supply voltage, the speed of separation, the characteristics of the load (resistive, capacitive, inductive) as well as other factors.
Future automotive systems are expected to utilize 42 volts in order to reduce the load currents and the associated wiring losses. This increased voltage could cause significant arc damage to occur to the present connectors designed for 12-volt operation. To avoid the possible liabilities associated with catastrophic connector failure, automotive manufacturers are requesting a new connector design that can be hot-swapped some significant number of times. Ten cycles is considered to be a minimum requirement.
To disconnect 42-volt power without significant damage requires interrupting about 1500-watts for many loads and as much as 15 KW for the main battery circuit. Present day modules used in automotive applications can consume more than 500 watts. Power supplies must deliver one or more kilowatts of energy. Conventional solutions require either that the current be shut off before the contacts are separated or unmated or employ a sacrificial contact portion. The cost, space, reliability, safety, performance and complexity of these conventional solutions make them unsuitable for many applications, including automotive electrical systems.
There are many things known in the power utility profession that will quickly extinguish an arc and there are many things known in the relay industry that will minimize arc damage to connectors and contacts. These can be found in literature, such as Gaseous Conductors by James D. Cobine and the Ney Contact Manual by Kenneth E. Pitney. Most of these methods are not practical in smaller and separable electrical connectors such as those used in automobiles, computers and appliances. None of the methods provided in the literature will eliminate arcing. Conventional contacts will be destroyed when rated currents are interrupted often enough and slowly enough, even though these conventional contacts may rated for current interruption. There is a finite life for existing connectors since arcing will occur and cause damage each time the connector is disconnected under load.
U.S. Pat. No. 4,079,440 disclosed the use of an impedance element between a long and a short contact to avoid an arc and consequent damage to the short contact. The impedance element can be a fixed capacitance and an inductance is included to limit inrush current. It is suggested that a resistance or a resistance in series with a capacitor could also be used as an impedance element. U.S. Pat. No. 4,681,549 discloses the use of a current limiting resistor between long and short pads on a printed circuit board. The use of a constant impedance, capacitance or resistance in this manner will tend to limit or suppress an arc in only limited circumstances. Fixed capacitors and resistors are only suitable for a relatively small range of currents and voltages. An electrical connector will typically be used for a much larger range of currents and voltages than can be practically accommodated by a fixed capacitor or a fixed resistance, which may prevent or suppress arcing for only a portion of the applications in which an electrical connector will be used.
Positive Temperature Coefficient Resistance (PTC) Devices, resistors or switches have been used, or suggested for use, in circuit breakers that are used to break fault currents, specifically defined and excessive overcurrents, for which these circuit breakers are rated. On the other hand, electrical connectors are expected to carry a wide range of currents during actual use. Even though an electrical connector may be rated to carry a specific current, in actual practice, an electrical connector will carry currents over a large range due to variations in the load. The cost, size and weight of an electrical connector will generally increase with increasing current rating, so the lowest rated connector suitable for use in a specific application will normally be used. Because multiple loads with different current needs pass through a single connector, as well as for economic, inventory and connector product line consistency, it is not uncommon to minimize the number of different connectors utilized in a specific product. The net result, is that a specific connector will carry anywhere from its rated current, or even an overcurrent for safety and life testing, to some significantly lower current. If that connector is to be disconnected while carrying a current, or hot swapped, without arcing, arc prevention must be effective for a large range of currents, starting from the arc threshold current to the rated current for that connector. In other words, unlike circuit breakers, hot swapped connectors must be protected from arcing over a wide range of currents. Therefore use of a PTC resistor in the same manner as it is used in a circuit breaker will not be suitable for use in an electrical connector. The trip time varies for a PTC device in which resistance is dependent upon the temperature of the device, and the temperature is dependant upon current because of I2R heating. Thus the trip time for a PTC device used in an electrical connector will vary because of the wide range of currents that will be carried by a particular electrical connector.
When PTC resistance devices are used in switches, relays, fuses and circuit breakers, both halves of electrical contacts remain within the same physical device. The contacts separate from each other, but only by a well defined and fixed distance, and the separated contacts are still part of the device package. The essential function of electrical connectors is to totally separate the two contact halves. No physical connection remains between the two halves, and all physical ties are broken between two mating connector contacts. In order to protect separating electrical contacts that are carrying arc-producing power, the PTC device must be connected across the contact pair until the current is sufficiently reduced to prevent arcing. Thus, the problem is that a physical electrical connection to both halves of the separating electrical contact must be maintained in a conventional use of a PTC device yet, in a connector, all physical connections must be broken.
In switches, relays, fuses and circuit breakers, where prior art PTC devices are used; the distance of contact separation and the rate of separation are controlled. In these prior art devices, the contact separation needs to only be enough to hold off the rated voltage. The rate of separation can be made as fast as possible to shorten the time in which arcing could occur, therefore minimizing any associated damage. Electrical connectors must be completely separated. Electrical connectors are also manually separated, and the rate of separation varies widely for existing electrical connectors. Even for a specific manually separated electrical connector design, the rate of separation will vary significantly each time two electrical connectors are manually unmated.
To overcome these problems, the instant invention preferably employs a positive temperature coefficient (PTC) resistor in an electrical connector in series with an auxiliary electrical contact portion or contact terminal, the combination of which is in parallel with a main electrical contact portion or contact terminal, which disconnects first. This arrangement of components parts will prevent arcing when two electrical connectors are unmated while carrying current. Both the main and the auxiliary contacts are matable with a terminal or terminals in a mating electrical connector. In the preferred embodiments, the main and auxiliary contacts are male terminals or blades that mate with a female or receptacle terminal in the mating electrical connector. However, the PTC resistive member could also be employed with the female terminals. The PTC resistive member should, however, only be employed with the terminals in one half of a mating pair of electrical connectors. The main or auxiliary contact portions or terminals in one of the two connectors must incorporate the PTC member. When a conventional discrete PTC member, such as a commercially available POLYSWITCH(copyright) device, is used, the main and auxiliary contact portions or terminals in the other of the two mating connectors must be connected together directly, with no discrete PTC device between them. However, in other applications the PTC means may be located in both connectors.
A discrete PTC resistive member can be employed into the main and auxiliary contact terminals so that the PTC device can form an integrated unit. One means for forming such an integrated unit would be to mold a PTC conductive polymer between the main and auxiliary contact terminals. The PTC conductive polymer could also be overmolded around portions of the main and auxiliary contact terminals, with the PTC conductive polymer being molded between the main and auxiliary contact terminals. Insert molding techniques could be used to position the PTC conductive polymer between, the main and auxiliary contact terminals. The PTC conductive polymer could also be a discrete component that is molded as a shape that would conform to parts of the main and auxiliary contact terminals and this discrete component could be bonded between the main and auxiliary contact terminals using solder, a conductive adhesive or some other conductive bonding agent.
The main contact should unmate before the auxiliary contact, and in the representative embodiments depicted herein, the auxiliary contact is longer than the main contact. In the preferred embodiment, the PTC member comprises a conductive polymer member in which conductive particles are contained within a polymer matrix. Normally the conductive particles form a conductive path that have a resistance that is larger than the resistance of the main terminal so that under normal mated operation, the main contact would carry substantially all of the current. However, as current increases in the PTC member, the polymer expands and the resistance increases. When current through the PTC member increases rapidly due to disconnection of the main contact terminal, the resistance will increase rapidly due to I2R heating of the polymer. To prevent arcing when the main contact is unmated, the disconnect time for the main contact must be less than the time for the resistance of the PTC member to increase too greatly. Most of the current through the main contact must be carried by the PTC member and the auxiliary contact until the main contact has moved to a position in which arcing is no longer possible. Before the auxiliary contact is disconnected from the mating terminal, the resistance in the PTC member must increase so that the current flow through the auxiliary contact will drop below the arcing threshold before the auxiliary contact is unmated. This time is called the trip time of this PTC resitive member. Since the trip time of the PTC member will depend on the initial current through the main contact, which can vary over a wide range, the trip time for a given electrical connector will therefore not be constant. To insure that the PTC member will trip, the electrical connector of this invention employs latches that cannot be activated, after the disconnection of the main contact, for a time interval that will be greater than the maximum trip time for the PTC member. However, these latches must also permit rapid movement between the two electrical connectors as the main contact moves through a portion of its path in which it is susceptible to arcing. Similarly, the auxiliary contact must move rapidly through an arc susceptible region as it is disconnected. The preferred embodiments of this invention therefore use multiple sets of latches that must be sequentially disengaged, and which provide a time delay between disconnection of a first set of latches and the disconnection of a second set of latches. This time delay should be longer than the maximum PTC trip time. This multiple latch configuration provides a versatile implementation of the invention. If, however, a specific electrical connector serves loads with a small difference between maximum and minimum current loads, a simpler latch mechanism can be utilized. The maximum achievable parting velocity and the added length of the auxiliary contact could in some cases provide adequate time for the PTC device to trip.